The mineral celestite contains strontium sulfate (SrSO.sub.4) which is the main source for the production of strontium carbonate, from which other strontium chemicals are obtained, e.g., strontium nitrate, strontium chloride, strontium hydroxide, etc. The major use of strontium carbonate is in the manufacturing of color television picture tubes, which contain about 5% to 7% of strontium oxide (supplied as SrCO.sub.3). The second and third major applications of the strontium carbonate are in the manufacture of ceramic ferrite magnets and in the purification of the zinc electrolite for the electrolytic production of zinc. Strontium carbonate is also employed as an ingredient of certain ceramics used for china and electrical glazes. The second most important strontium compound, obtained from the strontium carbonate, is the nitrate which is used in the fabrication of military and civilian signal flares and in pyrotecnics. Other strontium compounds which have less demand are strontium chromate, strontium phosphate and strontium chloride which are used as constituents of pigments, fluorescent lights and tooth pastes, respectively. As technology needs grow, it is expected that the application for strontium containing materials will increase, thus demanding a more efficient use of lower grade mineral resources through processes that meet quality demands and impose a negative impact on the enviroment.
The most common commercial process for producing strontium carbonate from celestite ores is the "black ash" process in which the celestite concentrate is calcined at elevated temperature, about 1000 degrees Centigrade, with finely ground carbon to produce strontium sulfide (SrS), carbon dioxide (CO.sub.2) and carbon monoxide (CO). The strontium sulfide is later leached with hot water and separated from solid impurities by filtration. The dissolved strontium sulfide is reacted with sodium carbonate, carbon dioxide or both to precipitate the strontium carbonate. Eventhough the calcining process is the most widely used, it has the disadvantages of (1) producing substantial amounts of strontium carbonate during calcining (which are lost as insolubles later during the leaching stage), (2) being energy intensive and (3) producing undesirable polluting by-products such as sodium sulfide (Na.sub.2 S, produced when sodium carbonate is employed as the carbonating compound) or hydrogen sulfide (H.sub.2 S, produced when carbon dioxide is used as the carbonate source).
Alternatively, strontium carbonate can be produced directly in a metathesis reaction by digesting relatively high grade celestite ore with soda ash solutions at low temperatures. Although simpler than the black ash process, the one stage direct conversion method has two main drawbacks: long periods of time are required in order for the reaction to be completed and little purification takes place so high purity products can not be obtained if low and medium grade celestite ores or concentrates are used. When high purity strontium carbonate (greater than 98.4%) is needed, the crude strontium carbonate obtained should be subjected to additional purification steps. The purification may consist of calcining the carbonate at about 1400 degrees Centigrade to obtain strontium oxide; the oxide is dissolved in water to produce SR(OH).sub.2 and the filtered solution reacted with CO.sub.2 gas to precipitate strontium carbonate of higher purity. Another method of purification consists of dissolving the strontium carbonate in an HCl solution; after the solution of the carbonates is completed the pH of the liquor is neutralized to cause the precipitation of the Fe impurities and remove them by filtration, together with the insoluble silica. The strontium present in the solution as SrCl.sub.2 can then be recovered by a second precipitation with soda ash.